This invention relates to push-pull power amplifier circuits. More particularly, it is concerned with transistorized push-pull amplifier circuits having compensation for turn-off delays caused by transistor charge storage.
Complementary transistor push-pull power amplifiers for providing a squarewave drive output are well-known. The output power transistors typically operate alternately in saturation. However, there is a delay in turning a transistor off because of the charge which becomes stored in the transistor when it operates in saturation. Thus when a transistor is turned on, the other transistor remains conductive due to the stored charge and both output transistors are on at the same time for a portion of each cycle. A large surge current is drawn from the power source and the power dissipation of the circuit is greatly increased. Also there is the possibility of hot spots developing in the transistors or that the transistors will fail due to secondary breakdown.
Various techniques have been developed to avoid the problem of charge storage delay in turning off the output transistors of push-pull amplifiers. In one arrangement the output transistors are transformer coupled. In another arrangement base biasing networks are used to sweep out the stored collector-base charge by the rapid application of a reverse bias to the base electrodes of the output transistors. The base biasing networks require the use of additional bias voltages and also consume additional power thereby reducing the efficiency of the circuit. In addition, neither of these two techniques completely eliminates simultaneous conduction in the output transistors over a wide range of output loads.
In another technique a fixed delay is introduced into the turn-on time of each transistor by the use of resistance-capcitance time constants or digital delay circuits. However, a fixed delay limits the circuit to narrow range of operating frequencies and to a constant load since the turn-off delay, assuming a fixed drive level to the output transistor, varies inversely with the output load.